An immunoconjugate consisting of a human monoclonal antibody directed against the deletion-mutant of epidermal growth factor receptor, EGFRvIII, conjugated via a non-cleavable linker to the cytotoxic agent maytansinoid DM1, with potential antineoplastic activity. The monoclonal antibody moiety of this immunoconjugate binds to EGFRvIII on tumor cell surfaces. After internalization, the DM1 moiety binds to tubulin, thereby disrupting microtubule assembly/disassembly dynamics and inhibiting cell division and the proliferation of cancer cells that express the EGFRvIII mutant. EGFRvIII, a deletion mutation of exons 2-7 in the epidermal growth factor receptor gene, is overexpressed by a variety of cancers, including glioblastoma multiforme, non-small cell lung carcinoma, and breast carcinoma.